This invention is related to solar insolation concentrators and particularly to a solar insolation concentrator formed from an assembly of flat reflector facets positioned within a slab-like substrate and a tool and process for fabricating this type of solar insolation concentrator.
The vast majority of solar insolation concentrators that have been developed have generally parabolic shapes or have pieces with partially parabolic shapes. A parabolic shape is mathematically desirable because when incoming solar energy is parallel to the central axis of a parabolic concentrator, the energy reflected off the surface of the concentrator is redirected toward a single focal point, where the maximum concentration of solar energy takes place. Because solar energy received from the sun is not uniformly parallel (because the sun is not a true point light source), the maximum energy flux of the concentrated solar energy will be found in a disk or torus shaped solar image centered about the focal point for the concentrator. This concentrated solar energy can then be absorbed by a solar receiver having a sufficient opening for the solar image and utilized, such as by a Stirling cycle engine.
Reflectors, such as those found in reflecting telescopes, have been developed that have extremely precise optical characteristics and very high reflectivity. Significant difficulties have been encountered when attempting to use the types of manufacturing techniques used to manufacture these types of reflectors to manufacture parabolic concentrators with the large dimensions needed to collect commercial quantities of solar energy.
Laboratory reflectors are typically manufactured from a single piece of substrate, often glass or metal, which is shaped precisely to the required configuration. The use of a single piece reflector is obviously impractical if the desired solar insolation concentrator has a very large diameter, such as 10 meters or 15 meters, as is commonly found in solar energy collection systems. A parabolic structure this large generally requires an assembly of components, such as steel tubing or steel truss members, which are fabricated into a parabolic-shaped framework. While the fabrication of this type of a parabolic framework is simple in concept, in practice it is very labor intensive and involves substantial manufacturing costs. In addition, it is virtually impossible to manufacture the parabolic framework with sufficient precision to allow the reflectors to be simply mounted and aligned with respect to the framework without additional individual adjustment and tuning of the reflectors to obtain a common focal point and acceptable concentration efficiencies.
Selecting the appropriate reflective material and method for mounting the reflectors to this framework is also problematic. A single-piece glass mirror with a diameter of 10 meters would obviously be prohibitively expensive to manufacture. The most efficient large solar concentrators in use today utilize arrays of curved glass mirror panels, with each panel being approximately 20 inches by 20 inches. To obtain a common focal point for the assembly of panels in the concentrator, each mirror has to be individually adjusted and shaped.
Concentrators utilizing single polished metal membrane mirrors have also been developed. In these concentrators, a single sheet of membrane material is stretched over a circular frame and a vacuum developed on the back of the membrane which draws the material into a parabolic shape. The reflection efficiency of this membrane material, however, is substantially less than the reflection efficiency of glass mirrors. It is also difficult to stretch the membrane into the precise parabolic shape required and difficult to keep the membrane fixed in this position for long periods of time.
Another approach is to divide the parabolic reflector surface area into a number of cells (for instance 12 or 16) and then use identical partially parabolic reflector facets for each of the cells. To obtain a common focal point, each reflector facet has to be individually adjusted after it has been attached to the framework. Because the shapes of these reflector facets are not individually customized to take into account their actual placement location within the parabolic reflector surface, the reflector facets do not have ideal optical characteristics when they are mounted in this sort of cellular array.
Plastic mirrors have been used, but these mirrors are significantly more easily damaged by exposure to the elements than glass mirrors and tend to rapidly lose their reflection efficiency.
An object of the present invention is to manufacture a solar insolation concentrator that does not require a parabolic framework to support individual reflectors and which incorporates a slab-like rigid substrate that fixes and maintains the reflectors in positions which have a common focal point.
A further object of the present invention is to manufacture a solar insolation concentrator which utilizes readily available, durable, efficient and relatively low cost flat back-silvered glass mirrors as reflector elements.
Another object of the present invention is to simplify and automate the process of fixing individual reflectors to a support structure and properly aligning the reflectors with respect to a designed focal point during the fabrication of solar insolation concentrators.
Still another object of the present invention is to reduce both the variable and fixed costs of manufacturing high efficiency solar insolation concentrators, including the costs of the materials, the labor required, and the tooling needed to fabricate the concentrators.
The inventive solar insolation concentrator is fabricated by fixing a multitude of flat solar reflectors to a rigid lightweight slab-like substrate that has inclined surfaces in its face surface that allow the reflectors to be aligned with a common focal point. The solar reflectors typically comprise small pieces of flat back-silvered glass mirrors having relatively thin glass. The pieces of glass mirror are preferably identically sized and may, for example, be two inches by two inches square. The substrate typically comprises a block of rigid lightweight strong material, such as a foamed plastic. High density cellular polystyrene (such as material sold under the "Styrofoam" trademark) has proven to be an acceptable substrate material for the inventive solar insolation concentrator.
The solar insolation concentrator is fabricated by first forming inclined surfaces in the substrate, typically by impressing a hot plate into the substrate material. Efficiently and accurately orienting this plate as it forms the impression in the substrate and then orienting a reflector as it is being fixed to the impression has proven to be a surprisingly difficult manufacturing problem. A typical engineering approach would be to mathematically calculate the proper inclination angle for each individual reflector position in the array and then to painstakingly position each reflector at its designated position and inclination angle using measuring devices such as rulers and levels. This type of labor intensive/brute force approach, while often acceptable for producing prototypes or research models, is not a cost effective method for mass producing a commercial product. It is extremely tedious and difficult to precisely form inclined surfaces in and adhere reflectors to a substrate that may require hundreds or thousands of such inclined surfaces and reflectors. Another commonly used approach would be to develop tooling, such as jigs, based on these mathematical calculations, that allow individual reflectors (or reflectors in symmetrically placed positions within the concentrator array) to be accurately positioned in the substrate in a repeatable manner. While better at cost effectively producing a commercial product than the first approach, this approach requires a substantial amount of tooling (i.e. individual tools are required for each individual reflector or individual symmetrically placed reflector group) and the tooling would only be useable to manufacture a single concentrator design. Another method would be to use robotic equipment programmed with the desired inclined surface coordinates. Robotic equipment having the required range of movement and degree of precision is both extremely expensive to purchase and relatively difficult to operate.
Applicant has developed an elegant solution to this problem which consists of a relatively simple tool that allows inclined surfaces to be made into the substrate which are precisely positioned and which assures that the reflectors adhered to the substrate have a common focal point. This fabrication tool is easily adjustable and can be used to develop a wide varieties of concentrator designs. The fabrication tool offers a degree of precision and repeatability in reflector placement that would be difficult to achieve by any other method.
The fabrication tool uses a pole, attached to a pivot that is fixed with respect to the substrate, and lower and upper pairs of guide rods, which are pivotally and slidingly connected to the pole. The lower guide rods are constrained by a guide fixed to the pole, through which the lower pair of guide rods are able to slide and also able to swivel about the fixed focal point. The upper guide rods are constrained by a sliding swivel located on the pole above the focal point through which the upper pair of guide rods are also able to slide. The sliding swivel is connected by a cable which passes over a pulley at the top of the pole and is connected to a weight that can slide within the pole. This weight constantly urges the sliding swivel toward the top of the pole. Ends of both pairs of guide rods are joined to components of the positioning head assembly. The guide rods are restrained by a thin cable connected to the end of the lower guide rods and the sliding swivel, which synchronizes the motion of the guide rods and assures that a reflector centered within the press plate of the positioning head assembly will share a common focal point with any other reflector similarly positioned on the substrate by the fabrication tool. The positioning head assembly also has a heating element that heats a press plate that is able to form inclined surfaces in the substrate into which the reflectors are placed.
The typical process for fabricating the inventive solar insolation concentrator using the fabrication tool is relatively simple. The substrate is first fixed with respect to the fabrication tool, such as by placing the substrate over the pivot and attaching the pole. If necessary, the fabrication tool is then adjusted to provide the desired focal length. If the fabrication tool is used to repetitively manufacture a single type of solar insolation concentrator, this adjustment step is not required after the first concentrator is manufactured. An inclined surface is then formed in the substrate by the positioning head assembly of the fabrication tool. An adhesive is then applied to the inclined surface and the back surface of a reflector is placed into contact with this adhesive. The reflector is then precisely positioned with respect to the desired focal point using the fabrication tool. This process is repeated until the face surface of the substrate is covered by reflectors (other than where the pivot and pole are located, if the pivot and pole are positioned within the substrate). The operator then takes any actions necessary to complete the concentrator, such as adding structural supports which increase the rigidity of the substrate.
It is also possible to spray the entire substrate surface with a thermoplastic adhesive (either before or after the inclined surfaces have been formed) and to attach the reflectors to the substrate by heating the reflectors before they are placed into contact with the adhesive. For certain types of substrate and reflector materials it may be possible to eliminate the adhesive altogether and to attach the reflectors to the substrate merely by warming the reflectors and pressing them against the substrate.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.